In Long Term Evolution (LTE) systems, a downlink HARQ feedback channel, i.e., Physical Hybrid ARQ Indicator Channel (PHICH) is used to indicate an uplink HARQ transmission per HARQ process for one User Equipment (UE). In order to reduce an uplink signaling overhead, there is a fixed timing relationship between PHICH and the corresponding uplink transmission for LTE systems.
In the fifth generation (5G) communication systems, such fixed timing configuration for the downlink HARQ feedback channel is not feasible for certain scenarios, such as unlicensed operations, since the downlink feedback channel may be not available due to the uncertain usage of unlicensed band. Therefore, an asynchronous HARQ mechanism will be adopted, in which a downlink HARQ feedback is dynamically handled by allocating uplink grants with the same HARQ process identity and a new data indicator (NDI), which is used to request uplink retransmission. The uplink grant is carried in the downlink control channel using certain Downlink Control Information (DCI) format, e.g., DCI format 0 in LTE systems.
The aforementioned downlink feedback via NDI is applicable for the scheduled uplink transmission, where the HARQ feedback relies on the continuous scheduling grant transmission using the carried NDI bit. However, for some other cases, e.g., a semi-persistent scheduling (SPS), a contention-based access or small data packet transmission, there is no continuous DCI for uplink data transmission, therefore, a DCI transmission only for downlink HARQ feedback may be necessary. For the small data transmission cases, there might be large amount of communication devices, therefore the overhead for transmitting one DCI only for HARQ feedback of one small data transmission could be quite large, which is undesirable for the communication efficiency of wireless systems.